1. Field of the Invention
The present invention relates to an activated sludge sewage treatment process and is particularly concerned with the removal from domestic and industrial wastewaters of nitrogenous pollutants as well as carbonaceous BOD.
2. Description of the Prior Art
In the well-known activated sludge system, domestic sewage, wastewater from industrial plants, or a combination of the two, is treated with air or other oxygen-containing gas in the presence of the micro-organisms furnished by the recycled activated sludge, to effect degradation of organic biologically degradable material, designated BOD. Following such oxidative treatment in one or more stages, the mixed liquor is introduced into a settler or clarifier, from which a portion of the settled solids are recycled to provide the activated sludge containing the microorganisms effective in consuming and digesting the organic matter (BOD) in the waste. The supernatant liquor from the settler or clarifier is returned to receiving waters generally after some further purification or disinfection treatment. While earlier commercial activated sludge plants employ air to furnish the oxygen needed to sustain the metabolic function of the microorganisms, more recent commercial installations employ, in one or more of the oxidation stages, aeration gas having a higher oxygen content than the 21% contained in atmospheric air.
A problem that has become more troublesome in recent years is that of eutrophication of streams, lakes and reservoirs receiving "purified" wastewaters which retain high nutrient content, chiefly such bio-stimulants as nitrogen and phosphorus. Effective means have been devised for removal of phosphorus compounds by chemical precipitation, carried out prior to, subsequent to, or during treatment to remove organic carbonaceous materials (BOD). Chemical usage required for effective precipitation is, however, substantial and costly. Removal of nitrogenous nutrients from wastewater has been the subject of extensive research in the past several years and a number of different methods have been suggested to achieve this result. Means of blowing ammonia out of alkalized wastewater has been found to be impractical in that the ammonia so removed is often recycled through the atmosphere to ground waters, hence ultimately to receiving waters. None of these or other suggested methods meet all of the desired goals from the standpoint of costs of installation and operation, facility and reliability of operation, and flexibility to meet variations in composition and quantity of influent waste.
Among the more promising methods for nitrogen removal from wastewater is that involving the so-called nitrificationdenitrification technique. Nitrification involves oxidation of ammonia values in the waste to nitrites and/or nitrates (referred to as NO.sub.x.sup.- by suitable microorganisms. As expressed in the article (inter alia), "Nitrification in the Activated-Sludge Process" by A. L. Downing, H. A. Painter and G. Knowles in the 1964 Journal of the Institute of Sewage Purification, pp. 130- 158, Nitrosomonas bacteria are effective in converting ammonia to nitrate and the nitrite is further oxidized to nitrate by Nitrobacter. These autotrophic nitrifiers utilize inorganic carbon for cell synthesis and obtain their energy from the oxidation of inorganic nitrogenous substrate. Since these nitrifiers grow at a slower rate than the heterotrophic bacteria responsible for the oxidation of the carbonaceous material in a biological waste treatment system, such as in an activated sludge process, it has heretofore been proposed to carry out the BOD removal from wastewater and the nitrification of ammonia values in separate treating stages, wherein the mixed liquor from the BOD removal stage is subjected to settlement and the sludge from such settlement reputed to be rich in bacteria effective in consuming organic carbonaceous material, is returned to the BOD removal first stage. The supernatant effluent from the solids settlement passes on to a second stage of aeration with oxygencontaining gas and the treated mixed liquor therefrom subjected to settlement of sludge solids which are returned to such second stage treatment, said settled sludge containing nitrifying microorganisms.
Exemplifying the two stage biological process once proposed as required for guaranteed complete nitrification, is the work described by H. E. Wild, C. N. Sawyer and T. C. McMahon at the October 1970 Water Pollution Control Federation Conference and published as "Factors Affecting Nitrification Kinetics", Journal WPCF 43, No. 9, 1971, pp. 1845- 1854.
Once oxidation of influent ammonia values is accomplished the product NO.sub.x.sup.- may still be harmful to receiving waters. Nitrate is a preferred substrate for algae growth. Furthermore, NO.sub.x.sup.- has been indicted as a causative precursor of methemoglobinemia (blue babies) as recently reviewed by H. I. Shuval and N. Gruener in the article "Epidemiological and Toxicological Aspects of Nitrates and Nitrates in the Environment", Am. Journal of Public Health 62 No. 8 (1972) pp. 1045- 1052.
In the absence of dissolved gaseous oxygen, numerous organisms such as Pseudomonas denitrificans will seek the nitrate oxygen as an oxidizing agent, i.e. a formal electron acceptor.
Facultative heterotrophs abound in activated sludge, and processes have been devised, and praised (Water Wasteland, by D. Zwick and M. Benstock of Ralph Nader's Study Group on Water Pollution, Chapter 19, page 374, Bantam Book, 1972), in which sludge is contacted with nitrate-reducing organisms to reduce nitrate to innocuous nitrogen gas.
Stoichiometric equivalents of oxygen required for nitrification and the oxygen demands necessary for denitrification may be simply summarized as: EQU Nitrosomonas NH.sub.3 + 3[0].fwdarw.H.sub.2 0 + HNO.sub.2 (Eq. 1) EQU Nitrobacter HNO.sub.2 + [0].fwdarw.HNO.sub.3 (Eq. 2)
and overall, EQU NH.sub.3 + 20.sub.2 .fwdarw.HNO.sub.3 + H.sub.2 O (Eq.3)
wherein the stoichiometric ratio of O/N is 4.57 g/g. Denitrifying bacteria exhibit the following stoichiometry: EQU 2NHO.sub.3 .fwdarw.N.sub.2 + H.sub.2 O + 5[O] (Eq.4)
wherein the ration of O/N is 2.86 g/g.
Due to the valence stage change in N only 62.5% of the oxygen donated to ammoniacal N may be considered as an electron acceptor in denitrification. The oxygen demanding substances biologically oxidized utilizing nitrate oxygen may be represented by the generic term biochemical oxygen demand (BOD) determined in accordance with procedures outlined in "Standard Methods for the Examination of Water and Wastewater", 13th Edition, A.P.H.A., A.W.W.A. and W.P.C.F. Publishers, 1971, for following initial cell utilization of dissolved gaseous oxygen or nitrate oxygen there appear to be similar metabolic pathways for oxygen utilization.
Exemplifying a sequential combination of techniques described above is the so-called "Three Sludge System" which employs separate stages of carbonaceous BOD removal, nitrification and denitrification, each stage having a subsequent settling step with independent recycle of sludge solids to its associated treating stage. The "Three Sludge System" is described by Mulbarger, M. C., in Journal WPCF, 43, No. 10, pp. 2059 et seq. (Oct. 1971).
In these described nitrification-denitrification procedures for removal of nitrogenous nutrients from wastewater the nitrification zone or stage is operated under aerobic conditions while the denitrification, effecting conversion of the previously formed nitrates and nitrites to free nitrogen, is carried out under anaerobic conditions. During nitrification, some part of the organic nitrogen substances may be converted into ammonia nitrogen and thus become available for conversion to nitrite or nitrate. For effective microbiological reduction of nitrates and nitrites by anaerobic denitrifying bacteria, sufficient organic sources of assimilable carbon, i.e. BOD, must be available to provide for their energy and growth.
Since, in a typical activated sludge plant most of the available BOD will have been oxidized prior to the denitrification operation, it has generally been found necessary to add an external assimilable carbon source, such as methanol or the like. Specifics of methanol utilization may be found described in the EPA Clean Water Water Pollution Control Research Series 17010DHT09/70 entitled "Methanol Requirement and Temperature Effects in Wastewater Denitrification".
Among problems encountered in the operation of certain activated sludge plants is that resulting from bulking sludge. Air activated plants often operate under such conditions of poor sludge characteristics. Oxygen activated sludge plants removing BOD, or nitrifying in addition to removing BOD, are not immune to the problem, as reviewed by R. C. Brenner in the article "EPA Experiences in Oxygen Activated Sludge", prepared in October 1973 for the EPA Technology Transfer Design Seminar Program. Furthermore, activated sludge plants designed to remove BOD, nitrify, and denitrify, such as the EPA operated Blue Plains facility (D. F. Bishop et al, "Single Stage Nitrification-Denitrification", October 9, 1974, seventh Annual WPCF Conference, Denver, Colo.) have suffered bulking sludge. Because of the poor settling characteristics of this kind of sludge, not only may an extreme reduction in the biomass concentration of the recycled sludge needed for effective operation of the activated sludge process result, but also a significant amount of the activated sludge may be lost over the weir of the settling tank. The bulking of sludge has been attributed by various investigators to different factors, and different techniques have accordingly been suggested for counteracting or preventing this bulking tendency. One of the important factors recognized as effecting or contributing to sludge bulking is the presence of filamentous microorganisms in the sludge. A solution to the problem of selective proliferation of filamentous bacteria in an activated sludge wastewater treating system is offered in U.S. Pat. No. 3,864,246. As described in this patent, the characteristics of the sludge are controlled by operating the initial contact zone for mixing influent waste with oxidizing gas and recycled sludge under conditions favoring selective propagation of active, rapidly settling, non-filamentous biomass thereby inhibiting the development of the undesired filamentous microorganisms. These favorable conditions for producing non-bulking sludge entail the provision of an adequately high food to biomass ratio in the initial mixing stage and the presence of sufficient dissolved oxygen in that stage.